The performance of electromagnetic antennas is measured with respect to the distance in a given direction or set of directions over which a given amount radio frequency (RF) power applied to the antenna's input terminals can propagate while having a signal strength above a given threshold. Performance is also measured with respect to the frequency bandwidth over which this can occur. An antenna comprises a collection of radiating elements which convert electrical energy to radiating photons, and the geometry and size of these elements determine the intrinsic radiation pattern of the antenna, representing the distribution of radiated power as a function of angular orientation with respect to the coordinate system in which the antenna is located. The radiation pattern indicates the ability of the antenna to concentrate energy along a given direction or set of directions, and the orientation of the peak of the radiation pattern gives the direction over which the propagation distance in free space will be greatest. The efficiency of the radiation process--i.e. the process of converting electrical energy to radiating photons--is dependent upon the operating frequency and is measured by what is termed here a radiation bandwidth. An antenna also exhibits a frequency dependent complex impedance at its input port or ports which affects the ability of the antenna to absorb power from a given source. This frequency dependency of the input impedance is characterized by the antenna's input impedance bandwidth. The net bandwidth of the antenna is dependent upon both the radiation bandwidth and the impedance bandwidth. An electrical matching network is generally placed between the antenna input port and the feed source to match the impedance of the antenna to that of the power source so as to maximize the amount of real power conducted into and absorbed by the antenna. Some of this absorbed real power is converted to heat due to ohmic losses in the conductive elements comprising the antenna, while the remainder is radiated by the antenna. The impedance bandwidth at the input to the matching network is generally different from that of the antenna. The performance of an antenna is dependent upon the ability of the antenna to absorb electrical energy conducted into the antenna input port, as indicated by the input impedance and impedance bandwidth, and upon the ability of the antenna to convert the conducted electrical energy to radiating photons, as indicated by the radiation pattern and radiation bandwidth. In operation, the orientation of the antenna, and with that the antenna's radiation pattern, relative to that of a given receiving antenna, will affect the maximum propagation distance that can be achieved for a given communications link between the two antennas.
The direction of polarization of an electromagnetic wave is given by the direction of the corresponding electric field component. If the direction of polarization is fixed, the wave is said to be linearly polarized, while if the direction of polarization rotates about the axis of wave propagation, the wave is said to be circularly polarized. The arts pertaining to electromagnetic radiation and propagation generally recognize that electromagnetic waves of a given energy which are linearly polarized in a vertical direction relative to the Earth's surface, i.e. vertically polarized, will propagate farther than corresponding electromagnetic waves of other polarizations. Vertically polarized waves are commonly created with resonant dipoles, or grounded quarter wave monopoles, oriented along a vertical axis. For a dipole, the length of the antenna at resonance--the operating frequency for greatest efficiency--is such that the antenna supports one half of a standing wave. While propagating on or along the antenna structure, the wave is referred as a guided wave, and the guided wavelength is generally about 95% of the free space wavelength for an electric dipole. The length of a resonant quarter-wave monopole will be one quarter of a guided wavelength. The physical size, especially the length, of these resonant dipole and monopole antennas can be a significant disadvantage, especially at low frequencies and for applications requiring a portable, vehicular mounted antenna.
A number of alternative means have been devised for reducing the size, or more particularly the length, of resonant dipole or monopole antennas. When operated at non-resonant frequencies, and particularly at frequencies where the resonant dipole or monopole antenna is electrically short or small, i.e. where the physical length of the antenna is shorter than the corresponding half or quarter guided wavelength, the input impedance becomes complex and likely unmatched to the power source, thereby reducing the amount of power that can be absorbed by the antenna. Matching circuits can be used to compensate for this effect and to thereby increase the efficiency of electrically short antennas, and these matching circuits can comprise either passive or active electrical networks. A dipole or monopole antenna can also be constructed with helically wound conductors, whereby the resonance length is governed by the length of the wire and the velocity factor of the helical wave guiding structure, while the antenna length is governed by the overall, and generally significantly shorter, length of the helix. A plurality of electrically short dipole or monopole antennas may also be operated as a phased array to as to concentrate the radiation power in a given direction. The benefits of reduced size in these alternative configurations, however, are generally obtained with the disadvantage of either reduced gain, or increased complexity or cost.
A low profile, i.e. short, vertically polarized antenna would be useful for a number of applications. These applications include portable communications equipment, such as on air, sea and land vessels and vehicles; where the physical length of a protruding antenna could either adversely affect aerodynamic drag, interfere with obstacles, or be overly conspicuous. These applications could also include low frequency land based communications where the height of the antennas is hazardous to aircraft and undesirable to neighboring residents. These tall antennas are also expensive to build and to maintain.
The radiation from an electric dipole or monopole antenna results from the spatial distribution of electric currents associated with the associated standing current waves on the antenna structure. The electric currents oscillate along the linear path of the antenna, and the direction of electric current corresponds to the direction of polarization of the resulting associated radiated wave. Applying the principle of duality of electromagnetic fields, a vertically polarized antenna can also be constructed in principle by replacing electric current sources with their equivalent magnetic current sources, where magnetic current is proportional to the time rate of change of the magnetic flux density B. A loop of uniform magnetic current is roughly equivalent to a linear electric current, whereby the axis of the loop of magnetic current is coincident with the line defining the linear electric current. Therefore for duality with an electric dipole or monopole antenna, the corresponding magnetic loop would be located in a plane normal to the electric dipole or monopole antenna. For a vertically polarized dipole or monopole, the magnetic loop will be in the horizontal plane.
Magnetic loop currents can be created with toroidal helical structures. An elementary toroidal helix comprises a single helical conductor which follows a path along the surface of a torus. The defining toroidal surface has a major axis and a minor axis, and corresponding radii. The major axis is normal to the plane of the torus, while the minor axis forms a circle whose radius is equal to the major radius of the torus. The toroidal surface is then defined as that surface whose distance from the minor axis is equal to the minor radius of the torus. The resonance properties of the toroidal helical structure are related to the length of the conductor, and the geometry of is associated toroidal helix. The physical height of this structure, when oriented in a horizontal plane as necessary for vertical polarization, is governed by the minor diameter of the toroidal helical structure. Since this height is generally significantly smaller than the corresponding resonant half or quarter wavelength, this structure has a low physical profile relative to that of a corresponding dipole or monopole antenna.
The prior art teaches various applications of elementary toroidal helical antennas. Ham, J. M. and Slemon, G. R. in Scientific Basis for Electrical Engineering, John Wiley & Sons, N.Y., 1961, 303-305 illustrate the use of the electric field created along the major axis of an elementary toroidal helix for accelerating charged particles. U.S. Pat. No. 3,646,562 teaches the use of an elementary toroidal helical coil to couple RF energy into a live tree via the electric field created along the major axis of the elementary toroidal helical coil for purposes of using a tree as a large antenna. While simple in construction, a disadvantage of the elementary toroidal helix is that in addition to creating a loop of magnetic current, the elementary toroidal helix also creates an associated loop of electric current, whereby the combined effects of the electric and magnetic loop currents produces a composite radiation pattern which differs from that of an electric dipole, and more particularly the radiated field contains both vertical and azimuthal components.
U.S. Pat. Nos. 4,622,558 and 4,751,515; related Australian Patent Application 548,541; and Canadian Patent 1,186,049 have disclosed three different groups of embodiments--referred as groups of prior art embodiments, infra--for canceling the azimuthal component of radiation gain present in the elementary toroidal helical antenna.
The first group of prior art embodiments comprise a plurality of closed interconnected ring elements, which are based upon the modified contrawound helix disclosed for use in traveling wave tubes by Birdsall, C. K. and Everhart, T. E. in "Modified Contra-Wound Helix Circuits for High-Power Traveling-Wave Tubes," IRE Transactions on Electron Devices, ED-3 (October 1956), 190-204. A typical linear contrawound helix comprises two coaxial helical windings, the helical pitch senses of each which are opposite to one another. If the electric currents in the separate windings are in phase, called the symmetric mode of operation, then the associated axial magnetic fields created by the separate helical winding elements cancel one another, while the corresponding electric fields reinforce one another. If the electric currents in the separate windings are of opposite phase, called the anti-symmetric mode of operation, then the axial magnetic fields reinforce one another, while the axial electric fields cancel one another. When applied to traveling wave tubes, the contrawound helix is normally operated in the symmetric mode. The modified contrawound helix of Birdsall and Everhart comprises a single conductor disposed as a series of poloidal ring elements interconnected with axial bar elements. At resonance, this modified contrawound helix operates similar to a bifilar contrawound helix. The condition for this mode of operation is that the circumferential length of the ring elements be on the order of a half wavelength. The first group of embodiments utilize a series of four modified contrawound helical elements disposed on a toroidal surface, whereby each element is fed in phase from a common signal source, and whereby each element would operate in the anti-symmetric mode so as to create a loop of quasi-uniform magnetic current without an associated loop of electric current.
The second group of prior art embodiments utilize first and second substantially closed, elongated conductors helically wound in bifilar relation on same toroidal surface. The conductors in these embodiments are shown having a continuous pitch sense. A given pair of windings is shown fed at diametrically opposite points on the toroidal helical structure, and a phase shift network is described in conduction with an embodiment having four toroidal helical conductors that are wound in parallel with a common, continuous helical pitch sense.
The third group of prior art embodiments are image plane variants of first group of prior art embodiments, supra, sectioned along the plane of the minor axis of the toroidal structure and including an image plane coincident with the sectioning plane. These embodiments utilize the principle of electrical imaging whereby a conductive image plane creates the electrical equivalent to the mirror image of the physical antenna structure above the image plane.
The associated toroidal helical structure for all three groups of prior art embodiments is taught to be at least one guided wavelength in circumference. The associated teachings also describe how the antennas are sized for a given operating frequency according to the relations from Kandoian, A. G. and Sichak, W., "Wide-Frequency-Range Tuned Helical Antennas and Circuits," Convention Record of the IRE, 1953 National Convention, Part 2--Antennas and Communications, pp. 42-47 for the propagation properties of waves on linear helical structures based upon the results from infinite sheath helices. However, U.S. Pat. application Ser. No. 07/992,970, infra, discloses that these relations were found to be in error by as much as a factor of 2 to 3 when applied to the operation of bifilar contrawound helical elements. The design relations for a toroidal helical antenna structure are used to determine the size and helical pitch of the associated toroidal helix for a given frequency of operation. The first and third groups of prior art embodiments also have the implicit limitation according to the theory of modified contrawound toroidal helical structures that the circumference of the rings must be on the order of a half wavelength in order to operate as a vertically polarized antenna. Since the ring diameter establishes the antenna height, this can be a constraining factor for some applications.
The prior art teaches the use of edge-slot structures for creating omnidirectional vertically polarized radiation fields wherein, according to Garnier, R. C., Study of a Radio Frequency Antenna with an Edge-Slot Like Structure, Ph.D. Dissertation, Marquette University, 1987, UMI Order Number 8716862 (which references U.S. Pat. No. 4,051,480) a toroidal shell structure with an circular resonant peripheral slot gap is fed from a pair of central internal nodes from inside the shell. This results in poloidal conduction currents on the shell structure in series with a displacement current across the peripheral slot, in contradistinction to the toroidal helical structures for which the currents are conducted by toroidal helical windings and for which there are no gaps in series with the conductive elements and across which must flow displacement currents.
An improved toroidal helical antenna is disclosed in U.S. Pat. application Ser. No. 07/992,970 now U.S. Pat. No. 5,442,369. This antenna uses a bifilar contrawound toroidal helical winding divided into four equi-angular segments each segment of which is one quarter guided electrical wavelength in length, wherein the helical pitch sense is reversed across segment boundaries, the junctions at segment boundaries comprise feed ports, and where the signal is fed at each of the feed ports. A two segment embodiment with a circumference of a half wavelength is also disclosed, for which the signal is simultaneously fed at two feed ports. These embodiments utilize multiple parallel feeds and corresponding feed matching networks. The contrawound helical windings are operated in an anti-symmetric mode wherein the magnetic loop currents created thereby are reinforced, and the associated loop electric current components effectively cancel one another. This improved toroidal helical antenna theoretically has a pure linear radiation polarization along the major axis of the associated toroid form, with near omnidirectionality in the azimuthal plane, and is not constrained to having a poloidal circumference of approximately one half wavelength as required of toroidal antenna embodiments constructed with ring-bar style modified contrawound helix windings, supra.
The improved toroidal helical antenna, supra, however, requires multiple, parallel signal feeds which are more complex to match and tune than would be a single feed port, because the separate feed networks can influence the operation of the antenna and can interact with one another. Also, the four segment embodiment of this antenna is one electrical wavelength in circumference. The two segment embodiment, while only a half wavelength in circumference, also requires multiple simultaneous feeds and operates at a low impedance resonance condition which has inherently lower bandwidth than the high impedance resonance condition at which the four segment embodiment operates.
U.S. Pat. No. 5,734,353, the '353 Patent, teaches an electrically small contrawound toroidal helical antenna comprising a single conductor with two length portions in overlapping contrawound relationship to one another. Electrical currents in the individual length portions travel in opposite circumferential directions around the toroid, so that the net circumferential electric current around the toroid is effectively zero. However, because of the contrawound helical relationship, the associated circumferential magnetic current components created by the respective electric current components in each of the toroidal helical length portions reinforce, so that the resulting radiation pattern is similar to that of an electric dipole that was coincident with and centered along the major axis of the torus. In other words, the resulting radiation pattern is strongly linearly polarized in a direction parallel to the major axis of the toroid. Depending upon the construction of the antenna, particularly the aspect ratio of the underlying torus form and the number of helical turns, other polarization components may also be present.
The '353 Patent, incorporated by reference herein, teaches a schematic symbolism for representing generalized helical and generalized toroidal helical windings as solid or dashed lines, the former representing a left had pitch sense, the later representing a right hand pitch sense, wherein the axial direction of the associated magnetic current and the projected axial direction of the associated electric current are the same for a right hand pitch sense helix, and opposite for a left-hand pitch sense helix. The radiation pattern of an electromagnetic antenna can be related to the effective electric and magnetic current distributions created by the antenna. For example, a uniform ring of magnetic current with no associated electric currents corresponds to the radiated electromagnetic field distribution of an electric dipole antenna. Furthermore, a uniform ring of electric current with no associated magnetic currents approximates the radiation pattern of a "Smith Cloverleaf" antenna. The radiation pattern for a particular set of current distributions can determined by either simulation or measurement.
In an exemplary mode of operation, the antenna is operated at a frequency such that the circumferencial length of the antenna is one half of an electrical wavelength. The slow wave properties of the contrawound helix make the corresponding physical length shorter than the free space wavelength according to the associated velocity factor, which depends upon the associated underlying helix geometry.